# Crystal Structures and the Unit Cell

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• 0:05 Crystal Structure
• 1:37 Types of Crystals
• 3:18 Lesson Summary

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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

This crystallography lesson explores the structure and properties of crystals. You'll learn about the parts of a crystal structure, types of crystals and patterns created by unit cells.

## Crystal Structure

Have you ever looked at salt with a magnifying glass? What about sugar? What about diamond? What do you see when you look at them? You probably see crystals.

Most substances form crystals in the solid phase. Crystalline substances are made of a three-dimensional repeating design formed by smaller identical units. There are many different types of crystal shapes that substances can take, and their shape is dependent on their chemical makeup.

The total three-dimensional arrangement of particles of a crystal is called the crystal structure. The actual arrangement of particles in the crystal is a lattice. The smallest part of a crystal that has the three-dimensional pattern of the whole lattice is called a unit cell. This is the smallest repeating unit in the lattice. The edges of each unit cell connect, and the opposite faces are parallel.

Crystalline solids are made of atoms, ions or molecules. The atoms, ions or molecules are called lattice points. Scientists depict the lattice points with circles or spheres. The spheres are packed together either as a two-dimensional square array or a two-dimensional close-packed array. The two-dimensional arrays are stacked on top of each other to make a three-dimensional structure. The type of shape the crystal takes is determined by the way it is packed - close or square - and how many lattice points are touching. There are fourteen different types of unit cell shapes.

## Types of Crystals

Crystals are often divided into four categories based on the types of particles and the bonding between the particles. The four types are ionic crystals, covalent network crystals, metallic crystals and covalent molecular crystals.

Ionic crystals are made up of both positive and negative ions that are arranged in a regular, repeating pattern. The strong ionic bond between the ions in the crystal makes it very hard and brittle. Ionic crystals have high melting points and are good conductors of electricity. Salt is an ionic crystal solid.

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